Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops onto an image substrate from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly. For example, the printhead assembly and the image substrate are moved relative to one another and the inkjets are controlled to eject ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that selective activate inkjets to eject ink onto an image substrate. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form and delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink, which is supplied to a printhead.
During the operational life of an inkjet printer, inkjets in one or more of the printheads may become unable to eject ink in response to receiving a firing signal. The defective condition of the inkjet may temporarily persist so the inkjet becomes operational after one or more image printing cycles. In other cases, the inkjet may remain unable to eject ink until a purge cycle is performed. A purge cycle may successfully unclog inkjets so that they are able to eject ink once again. Execution of a purge cycle, however, requires the imaging apparatus to be taken out of its image generating mode. Thus, purge cycles affect the throughput rate of an imaging apparatus and are preferably performed during downtime.
In previously known imaging devices, a controller operated printheads to print a test pattern onto an image substrate. The test pattern was scanned with an optical sensor, which generated image data corresponding to the intensity of the light reflected by the bare image substrate and the ink on the image substrate. These image data are processed by the controller to identify the positions of the ink on the image substrate and from this positional information the controller can detect defective inkjets as well as printhead position data that can be used to adjust or compensate for erroneous printhead positions. This printer process, however, is sometimes unable to detect defective inkjets. In one situation that is problematic, an inkjet is able to print a sequence of drops to form a dash in a test pattern, but during printing operations, especially during the printing of high density coverage areas, the inkjet fails to eject ink. Consequently, these inkjets are not detected as being defective and no compensation technique is enabled to mask the inability of these inkjets to eject ink properly. Methods to detect sporadic inkjets reliably would be useful.